I. Field of the Invention
The present invention generally pertains to mixing drums for batch style tilt mixers particularly those used to mix erosive materials such as concrete prior to its being loaded into transit carriers including concrete mixer trucks, commonly known as readymix trucks. More specifically, the present invention relates to improvements in the drum design which enhance mixing efficiency.
II. Discussion of Related Art
Tilt mixers having drums of various types have been used for many years in the construction industry for mixing batches of concrete to be loaded into trucks which, in turn, transport the concrete to job sites for placing. Over the years, experience has shown it to be most economical to manufacture tilt mixers themselves in a size capable of being transported by truck over the highways and mounted for use at the mixing plant. Therefore, they must be within the maximum size which can be legally transported on a highway.
Tilt mixers are typically installed in an elevated position so that trucks hauling the pre-mixed materials may be loaded from above by tilting the mixer and discharging the mixed batch through a chute into the charging end of the truck.
Such devices typically are designed to be charged and to conduct the mixing operation in a substantially horizontal position. The charging end of the drum closely addresses a fixed loading chute for receiving measured amounts of aggregate, including sand and stone, cement and water, in the case of concrete, according to the batch formula used. The charging end of the drum must be configured so as to clear the charging chute when the drum is tilted to discharge the mixed materials into the discharge chute for delivery to the site of the pour. The, clearance with respect to the front or discharge end of the mixing drum is also important. Consideration of the height necessary to mount the drum for tilt loading of ready mix trucks and the clearance for the charging chute are important considerations which must be weighed together with the desire to make the drum itself as close as is reasonable to the ideal mixing shape, in which the length is the same as or approaches the diameter of the drum.
Most previous mixing drums of the class described have had to provide a cone with a severe angle to accommodate the charging chute and have generally been shaped with a reduced the diameter with respect to length. They have also had to use a pivot point which is removed some distance from the discharge end of the drum with respect to the axis about which the drum is tilted in order to reduce the torque required to tilt the drum for discharging materials. This had led, to the development of a general configuration for tilt mixer drums which has become somewhat of an industry standard. It includes ends having rather radical conical sections and a rather elongated mixing central zone. The drum is mounted to be pivoted at a distance from the discharge end. The drum itself has to be mounted at a relatively high position in order for the truck loading-unloading chute beneath the discharge end of the mixer to clear the tops of the trucks which drive under the mixer for loading as the loading system must accommodate the mixing drum when fully tilted to discharge the mixed material.
In addition to the need to decrease the length to diameter ratio of the mixing drum itself, the efficiency and uniformity of mixing have left much to be desired with respect to drum agitators. For example, there has existed a need to increase the efficiency or speed of mixing a batch with respect to obtaining proper uniformity from front to back of the mixing drum. While batches having local uniformity can be mixed fairly rapidly, the time required to achieve the desired total uniformity within the batch from front to back of the mixer could be greatly improved. Thus, a need has existed to increase the mixing efficiency of a rotary tilt mixing drum both from the standpoint of the shape of the drum and from the standpoint of improving the effect of the agitation means within the drum to achieve more efficient mixing.